A medium is a label, not a measurement. No standard defines it, so the same letter can describe noticeably different bikes. Compare stack and reach instead, and treat the size letter as a rough starting point at best. Canyon puts it bluntly in its sizing advice: “ignore size labels” because a medium in one brand can feel totally different in another.
Why the letters do not line up
Each brand picks its own scale. Canyon’s range runs from 3XS to 2XL depending on the model, its mountain bikes mostly come in five sizes from XS to XL, and its gravel bikes span 2XS to 2XL, which Canyon describes as wider than most brands offer. A brand with five sizes and a brand with nine will not put “medium” in the same place.
Even inside one company the letter shifts between bike types. Canyon notes that an M mountain bike is not necessarily the same as an M road or gravel frame. A gravel writer at the same company goes further: the equivalent of a medium on one maker’s bike could be a small or a large on another.
Why the old number, seat-tube length, stopped helping
Older frames had horizontal top tubes, so seat-tube length hinted at stand-over height and, roughly, at how long the top tube would be. Cycling UK points out that sloping top tubes are now common, and the seat-tube length alone tells you little about whether a bike will fit. Its example: you are as likely to fit a 51 cm frame as a 54 cm one. Charts built on seat-tube length only work when they are specific to that one model.
The two numbers to compare: stack and reach
Both are measured from the bottom bracket, the shell where the crankset spins, and both describe where the front of the bike sits relative to it.
- Stack is the vertical height from the bottom bracket to the top of the head tube. A bigger stack gives a more upright position.
- Reach is the horizontal distance from the center of the bottom bracket to the head tube. A longer reach stretches you out, with your hands further forward.
Because they are measured the same way on every frame, they let you compare a bike from one brand with a bike from another. Canyon recommends this as the most accurate way to match a bike you already own, especially when switching brands. If you have a bike that feels right, look up its stack and reach on the maker’s geometry chart and use those figures as your baseline. Our guide to fitting a bike properly covers what reach means once you are sitting on one.
Worked example: two mediums from one brand
Canyon’s gravel guide lists two of its own models, both in size medium:
| Model (size M) | Stack | Reach | Stack+ | Reach+ |
|---|---|---|---|---|
| Grizl CF SLX 8 Di2 | 579 mm | 402 mm | 666 mm | 461 mm |
| Grail CF SLX 8 | 591 mm | 411 mm | 669 mm | 456 mm |
The frames differ by 12 mm in stack and 9 mm in reach. Once the stem and spacers are included (the Stack+ and Reach+ columns), the picture flips on reach: the Grizl comes out 5 mm longer. That is the reason to read the exact column a chart provides. The plain frame numbers describe the frame alone, while the “plus” figures account for the supplied stem and spacers, so compare like with like.
How to use the numbers when shopping
- Find the stack and reach of a bike you already ride comfortably, from its maker’s geometry page.
- Open the chart for the new bike and look for the size whose stack and reach sit closest to yours. Use the same kind of figure on both sides: frame-only against frame-only, or complete-bike figures against complete-bike figures.
- If you land between two sizes, decide which way you lean. Cycling UK suggests going larger if you have a long torso for your height. Canyon’s advice for riders between sizes points the same way: a longer torso relative to leg length is a reason to size up.
- Remember what can be changed. Canyon says minor saddle, handlebar or stem adjustments can fine-tune the fit.
Limits of adjusting a bike that is the wrong size
Cycling UK describes the right size as one in the ballpark for your height and desired riding position, not an exact match. Parts can close a small gap but not a large one. A super-long stem can fix a too-short frame’s reach, but it makes steering “tiller-like” and slow. A tall stack of spacer washers can raise the bars, but Cycling UK warns that it might snap.
If a bike only fits you with an extreme stem or a tall tower of spacers, choose another size or frame instead. If handlebar or steerer parts look cracked, bent or loose, stop riding and have a bike shop inspect them before you ride again.
Bikes where the letter matters less
Some designs are deliberately forgiving. Canyon mentions unisize e-city bikes with upright, relaxed fits, and the same logic applies to other relaxed city bikes: a single frame can cover many riders. For those, test-riding or reading the maker’s height guidance matters more than chasing exact stack and reach. If you are still deciding what kind of bike to buy, start with which type suits a beginner, and for electric models see choosing your first e-bike.
What this article does not cover
Measuring your own body, inseam and height is covered by each brand’s own size tool. Canyon, for instance, asks for height and inner-leg length. Use the maker’s tool for the model you are buying, then use stack and reach to cross-check against a bike you know.
Sources
- Canyon: Canyon bike size guide: Road, mountain, gravel and more
- Canyon: Gravel bike sizing guide
- Cycling UK: How to choose the right size bike
Last reviewed: October 2026.